It is sometimes desirable to provide a switch between an input portion and an output portion of a microwave transmission line. One device for performing such a function is a radio frequency (RF) micro-electro-mechanical system (MEMS) switch. One example is the switchable capacitor disclosed in the U.S. patent application entitled “Switchable Capacitor” (U.S. application Ser. No. 13/556,273), filed on Jul. 24, 2012, the entire content of which is incorporated herein by reference. The RF MEMS switch typically has a substrate with two conductive posts spaced apart on the substrate. The switch includes a switchable capacitor having a conductive part (e.g., a bottom electrode of the switchable capacitor) on the substrate disposed between, but electrically separated from, the bottom portions of the posts. The bottom electrode is covered by a layer of a solid dielectric material. A flexible (e.g., elastic), electrically conductive membrane (e.g., the upper electrode of the switchable capacitor) extends between, and has ends thereof electrically connected to, the tops of the posts, so that a central portion of the flexible, electrically conductive membrane is suspended above the bottom electrode. An input portion of a microwave transmission line is coupled to an end of one of the top and bottom electrodes and an output portion of the transmission line is coupled to the other end of the one of the top and bottom electrodes. In some examples, the input portion of a microwave transmission line may be coupled to one of the top and bottom electrodes and an output portion of the transmission line may be coupled to the other one of the top and bottom electrodes.
The switch includes a switchable voltage source, coupled between the top and bottom electrode for producing a switchable electrostatic force between the two electrodes. The switchable electrostatic force changes the spacing between the two electrodes, and hence switches (e.g., modifies) the capacitance of the switch, selectively in accordance with a voltage produced by the switchable bias voltage source. The actuation of the switch from a low capacitance state (e.g., open state) to a high capacitance state (e.g., closed state) is prompted by a pull down voltage, which produces sufficient attractive electrostatic force to bring the top and bottom electrodes closer to one another.
In some applications, such as high power applications, it is desirable to have a high pull down voltage on the top, flexible electrode because if the pull down voltage is too low, the RF signal can itself activate the switch, which is an undesired effect. If the pull down voltage on a conventional RF MEMS switch is increased, this will increase the electric field strength in the solid dielectric when the MEMS switch is closed (i.e., in the activated state). This can result in solid dielectric breakdown or excessive charging of the solid dielectric which can lead to “stiction” of the second, flexible electrode which can cause the flexible membrane to remain in the down or closed position in the deactivated state.
What is desired is a bias scheme that reduces the electric field strength experienced by the dielectric, thus improving (e.g., increasing) the durability of the RF MEMS switch, without affecting its actuation responsiveness.